Widely separated binary systems of very low mass stars

In this paper we review some recent detections of wide binary brown dwarf systems and discuss them in the context of the multiplicity properties of very low-mass stars and brown dwarfs.Comment: 2 pages, 1 figure (new version with minor corrections); to appear in the proceedings of the workshop "Ultra-low mass star formation and evolution", to be published in A

Discovery of Radio Emission from the Tight M8 Binary: LP 349-25

We present radio observations of 8 ultracool dwarfs with a narrow spectral type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8 binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system detected in radio and its trigonometric parallax pi = 67.6 mas, recently measured by Gatewood et al., makes it the furthest ultracool system detected by the Very Large Array to date, and the most radio-luminous outside of obvious flaring activity or variability. With a separation of only 1.8 AU, masses of the components of LP 349-25 can be measured precisely without any theoretical assumptions (Forveille et al.), allowing us to clarify their fully-convective status and hence the kind of magnetic dynamo in these components which may play an important role to explain our detection of radio emission from these objects. This also makes LP 349-25 an excellent target for further studies with better constraints on the correlations between X-ray, radio emission and stellar parameters such as mass, age, temperature, and luminosity in ultracool dwarfs.Comment: accepted by ApJ, referee's comments included, typo in equation 1 correcte

SMA observations of the proto brown dwarf candidate SSTB213 J041757

Context. The previously identified source SSTB213 J041757 is a proto brown dwarf candidate in Taurus, which has two possible components A and B. It was found that component B is probably a class 0/I proto brown dwarf associated with an extended envelope. Aims. Studying molecular outflows from young brown dwarfs provides important insight into brown dwarf formation mechanisms, particularly brown dwarfs at the earliest stages such as class 0, I. We therefore conducted a search for molecular outflows from SSTB213 J041757. Methods. We observed SSTB213 J041757 with the Submillimeter Array to search for CO molecular outflow emission from the source. Results. Our CO maps do not show any outflow emission from the proto brown dwarf candidate. Conclusions. The non-detection implies that the molecular outflows from the source are weak; deeper observations are therefore needed to probe the outflows from the source.Comment: 7 pages, 4 figures, accepted for publication in A&

Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Sco OB association

Aims: Low-resolution optical spectroscopic observations for 78 very low-mass star and brown dwarf candidates that have been photometrically selected using the DENIS survey point source catalogue. Methods: Spectral types are derived for them using measurements of the PC3 index. They range from M6 to L4. H_alpha emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths are measured in the spectra. Spectroscopic indices of TiO, VO, CrH and FeH molecular features are also reported. Results: A rule-of-thumb criterion to select young very low-mass objects using the NaI doublet equivalent width is given. It is used to confirm seven new members of the Upper Sco OB association and two new members of the R Cr-A star-forming region. Four of our field objects are also classified as very young, but are not members of any known nearby young association. The frequency of lower-gravity young objects in our field ultracool sample is 8.5%. Our results provide the first spectroscopic classification for 38 ultracool dwarfs in the solar vicinity with spectrophotometric distances in the range 17 pc to 65 pc (3 of them are new L dwarfs within 20 pc).Comment: 25 pages, 10 figures, 7 tables, Accepted by A&

New neighbours VI. Spectroscopy of DENIS nearby stars candidates

We present spectra of 36 nearby star candidates and 3 red giant candidates, identified in the DENIS database by Phan-Bao et al. (2003). 32 of the dwarf candidates are indeed nearby red dwarfs, with spectral types from M5.5 to M8.5. Out of 11 targets with low proper motion (mu < 0.1$ arc-sec/yr) but a Reduced Proper Motion above an inclusive threshold, 9 are red dwarfs. The 4 contaminants are all reddened F-K main sequence stars, and they could have been eliminated by checking for some well known high latitude molecular clouds. These stars might be of interest as probes of interstellar absorption. For the red dwarfs we derive spectral types and spectroscopic distances, using a new calibration of the PC3 spectral index to absolute magnitudes in the I, J, H and K photometric bands. We confirm 2 new members of the 12 pc volume (2 new M8.5), and one M7.5 NLTT object closer than 10pc. We also show that one quarter of the stars with photometric distances under 30 pc have too small a proper motion for inclusion in the NLTT catalog.Comment: Accepted by A&

Detection of lithium in nearby young late-M dwarfs

Late M-type dwarfs in the solar neighborhood include a mixture of very low-mass stars and brown dwarfs which is difficult to disentangle due to the lack of constraints on their age such as trigonometric parallax, lithium detection and space velocity. We search for young brown dwarf candidates among a sample of 28 nearby late-M dwarfs with spectral types between M5.0 and M9.0, and we also search for debris disks around three of them. Based on theoretical models, we used the color $I-J$, the $J$-band absolute magnitude and the detection of the Li I 6708 $\AA$ doublet line as a strong constraint to estimate masses and ages of our targets. For the search of debris disks, we observed three targets at submillimeter wavelength of 850 $\mu$m. We report here the first clear detections of lithium absorption in four targets and a marginal detection in one target. Our mass estimates indicate that two of them are young brown dwarfs, two are young brown dwarf candidates and one is a young very low-mass star. The closest young field brown dwarf in our sample at only $\sim$15 pc is an excellent benchmark for further studying physical properties of brown dwarfs in the range 100$-$150 Myr. We did not detect any debris disks around three late-M dwarfs, and we estimated upper limits to the dust mass of debris disks around them.Comment: 10 pages, 5 figures, accepted for publication in Astronomy and Astrophysic

Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf

[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B~1.7 kG magnetic field. A quiescent non-variable component is also detected, likely due to emission from a uniform large-scale field. The H-alpha emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (vsini=27 km/s), and the binary orbital inclination (i=142 deg), to derive a radius for the primary star of 0.078+/-0.010 R_sun. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides a constraint on the mass-radius relation below 0.1 M_sun. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr.Comment: Submitted to Ap

First Direct Simulation of Brown Dwarf Formation in a Compact Cloud Core

Brown dwarf formation and star formation efficiency are studied using a nested grid simulation that covers five orders of magnitude in spatial scale (10^4 - 0.1AU). Starting with a rotating magnetized compact cloud with a mass of 0.22 M_sun, we follow the cloud evolution until the end of main accretion phase. Outflow of about 5 km/s emerges about 100 yr before the protostar formation and does not disappear until the end of the calculation. The mass accretion rate declines from 10^-6 M_sun/yr to 10^-8 - 10^-12 M_sun/yr in a short time (about 10^4 yr) after the protostar formation. This is because (1) a large fraction of mass is ejected from the host cloud by the protostellar outflow and (2) the gas escapes from the host cloud by the thermal pressure. At the end of the calculation, 74% (167 M_Jup) of the total mass (225 M_Jup) is outflowing from the protostar, in which 34% (77 M_Jup) of the total mass is ejected by the protostellar outflow with supersonic velocity and 40% (90 M_Jup) escapes with subsonic velocity. On the other hand, 20% (45 M_Jup) is converted into the protostar and 6% (13 M_Jup) remains as the circumstellar disk. Thus, the star formation efficiency is epsilon = 0.2. The resultant protostellar mass is in the mass range of brown dwarfs. Our results indicate that brown dwarfs can be formed in compact cores in the same manner as hydrogen-burning stars, and the magnetic field and protostellar outflow are essential in determining the star formation efficiency and stellar mass.Comment: 13 pages, 3 figures. Accepted for publication in ApJL. For high resolution figures, see http://www2-tap.scphys.kyoto-u.ac.jp/~machidam/astro-ph/BD.pd

Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM513-46546

[Abridged] We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM513-46546, with a duration of 9 hours. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with L_X/L_bol~10^-4.9, the faintest to date for any object later than M5, and a possible weak X-ray flare. TVLM513-46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, suggesting that, unlike in early M dwarfs, chromospheric heating may not be due to coronal X-ray emission. More importantly, we detect a sinusoidal H-alpha light curve with a period of 2 hr, matching the rotation period of TVLM513-46546. This is the first known example of such Balmer line behavior, which points to a co-rotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light curve peak during the four observed maxima. From the radio data we infer a large scale steady magnetic field of ~100 G, in good agreement with the value required for confinement of the X-ray emitting plasma. The radio flares, on the other hand, are produced in a component of the field with a strength of ~3 kG and a likely multi-polar configuration.Comment: 13 pages, 4 figure

Search for nearby stars among proper motion stars selected by optical-to-infrared photometry. II. Two late M dwarfs within 10 pc

We have identified two late M dwarfs within 10 parsecs of the Sun, by cross-correlating the Luyten NLTT catalogue of stars with proper motions larger than 0.18 arcsec/yr, with objects lacking optical identification in the 2MASS data base. The 2MASS photometry was then combined with improved optical photometry obtained from the SuperCOSMOS Sky Surveys. The two objects (LP775-31 and LP655-48) have extremely red optical-to-infrared colours ((R-K)~7) and very bright infrared magnitudes (K_s<10): follow-up optical spectroscopy with the ESO 3.6-m telescope gave spectral types of M8.0 and M7.5 dwarfs, respectively. Comparison of their near-infrared magnitudes with the absolute magnitudes of known M8 and M7.5 dwarfs with measured trigonometric parallaxes yields spectroscopic distance estimates of 6.4+/-1.4 parsecs and 8.0+/-1.6 parsecs for LP775-31 and LP655-48, respectively. In contrast, Cruz & Reid (2002) recently determined spectral types of M6 for both objects, and commensurately larger distances of 11.3+/-1.3 parsecs and 15.3+/-2.6 parsecs. LP655-48 is also a bright X-ray source (1RXS J044022.8-053020). With only a few late M dwarfs previously known within 10 parsecs, these two objects represent an important addition to the census of the Solar neighbourhood.Comment: Astronomy & Astrophysics (Letters), in press; 5 pages, 1 figure, uses aa.cls version 5.